Response:

This is an argument from incredulity.
Complexity only
indicates that something is difficult to understand, not that it is
difficult to evolve. Evolution, unlike design, is not constrained by
requirements for simplicity.

Brains come in many different sizes. The sea slug (Aplysia), for
example, has only about 20,000 neurons in its entire nervous system.
Coelenterates have an even simpler nervous system consisting of a nerve
net and nothing even close to a brain. There are innumerable
intermediate forms of brains between humans and brainless animals;
gradual evolution of the brain presents no challenge.